I believe this unit is designed for O and larger.
I suspect the fact that Tim has HO and Garden Scale models himself was a contributing factor.
from the manual:
The LM-3S-G can be installed in a G or O scale model train locomotive that you
want to control the motion, lights, and sounds with a RailPro controller.
Specifications
Maximum Power Input Voltage: 24 Volts
Maximum Motor Stall Current: 8 Amps
Maximum Output Current on a Single Output: 1.2 Amps.
Maximum Output Current on all Outputs Combined: 1.2 Amps.
Audio Output Power: 13 Watts into a 4 ohm load. 7.4 Watts into an 8 ohm load.